This invention concerns release sheets (films) useful in sheet molding compound applications, (SMC) thick (or "Takeda") molding compound (TMC) applications, or in bulk (or "dough") molding (BMC) applications. This invention also concerns sheet molding compounds made using the release sheets of this invention.
In various processes for producing sheet molding compounds, the process typically includes casting a layer of heat-curable thermosetting resin composition such as an unsaturated polyester composition containing styrene monomer, usually with chopped fiberglass and filler, onto a polymeric film as a release sheet and carrier sheet. Polymeric film is then also applied to the top surface of the heat-curable thermosetting resin composition to form a sandwich composition. The process includes passing the sandwich composition through a plurality of compaction rolls, and then winding the sandwich composition into a roll for partial curing and for later use, for example, in a compression molding system for pressure molded products. The polymeric release and carrier film is typically stripped off and disposed of prior to the compression molding step.
The polymeric carrier film provides a barrier for the particular monomer employed in the thermosetting resin, such as a styrene monomer, yet the film should be easily removable from the sandwich composite so produced. A polyamide polymeric film providing good barrier characteristics for the styrene monomer in the liquid resin is unsuitable for use, since the nylon tends to adhere to the styrene monomer heat-cured thermosetting resin, and is therefore difficult to remove.
A polymeric carrier film having release properties with low crystallinity has been proposed, with the polymeric film comprising a blend of a polyamide, such as nylon 6/66, together with an olefin of defined crystallinity, such as an ethylene vinyl acetate copolymer. Employment of the modified nylon carrier film is described, for example, in European patent application 0 027 191, published Mar. 21, 1984, which is hereby incorporated by reference. Desirable characteristics of such nylon modified support films in the manufacture of sheet molding compounds products has also been described in the publication "Nylon Support Films for the Manufacture of SMC Products," by P. Stuart Bollen et al, at the 35.sup.th Annual Technical Conference 1990, Reinforced Plastics Companies Institute, The Society of Plastics Industry, Inc., also hereby incorporated by reference. This publication sets forth the distinct advantages of the modified nylon carrier film with the employment of polyethylene (PE) film to support the formation of the SMC compounds. However, it was reported that PE film, at a low cost and per unit area, relatively inert and with non-adhesive properties, has been exceptionally permeable to the common volatile liquid aromatic hydrocarbons, such as benzene, toluene and styrene monomers, as employed in the heat-curable thermostatic styrene modified resins.
In addition, it may be appreciated that monomer components such as styrene are frequently employed in the resins as crosslinking agents. Undue loss of styrene may lead to useless batch of compound. Previously, the edges of the SMC and TMC have been styrene deprived because styrene could escape through the unsealed film, at the edges of the compound during the process and during maturation, after removal from the maturation area, and prior to molding. Trimming of styrene deprived edges can be a costly loss of compound to the end user.
It should be further noted that while it was possible previously to seal the edges of the film, the force required to pull the film apart was too great to do so without cutting of the film. Molders thus found that separation of the film from the SMC or TMC was inefficient and too expensive since special equipment was required. A need therefore exists for a film which can be sealed at the edges and then peeled apart so that molding compound can be used.
It is therefore desirable to provide for a new and improved, low cost and efficient carrier support film product for the manufacture of SMC, TMC, and BMC products in the processes for production thereof.